JP6448133B2 - Vacuum consolidation method and vacuum consolidation system - Google Patents

Description

  The present invention relates to a vacuum consolidation method and a vacuum consolidation system for improving ground by vacuum consolidation.

  A vacuum consolidation method is known in which ground improvement is performed by vacuum consolidation without laying an airtight sheet on soft viscous ground. In the vacuum consolidation drain method, as shown in FIG. 9, the plastic board drain material 11 is connected to one end thereof with an airtight cap 12 and a drainage hose 13, and placed on a soft ground G ′. The end is connected to a vacuum pump to improve vacuum consolidation (Patent Document 1). In this case, it is standard that the airtight cap 12 is installed at a depth of about 1 m in the ground so that the surface layer of about 1 m of the conventional clay layer is used as a negative pressure seal layer.

Japanese Patent No. 4493522

"Vacuum Condensation Drain Method Technical Data", page 8 Vacuum Consolidation Drain Method Study Group, published in March 2013 Kozo Miki “Permeability and Permeability Coefficient” Soil and Foundation Vol.19 No.6 3-4 Issued in 1971 Shinsha Hiroshi, Miyamoto Kenji, Watanabe Yoichi “Experimental Study on the Effect of Vacuum Consolidation in the Case of Suction Water from the Intermediate Sand Layer” Japan Society of Civil Engineers B3, Vol.69 No.2 pp. 970-975 2013

However, if the stratigraphic structure of the ground is complex and there is a mutual layer where sand and clay overlap each other on the surface layer of the ground, a negative pressure sealing layer with a thickness of about 1 m may not be secured in the surface layer. . In this way, when there is a mutual layer of sand and clay on the surface layer of the ground, it is conceivable that a low-permeability soil layer having a thickness of about 1 m is newly built on the ground to form a negative pressure sealing layer. The permeability coefficient required for the hermetic seal layer is about 10 ^-7 m / s or less (see Non-Patent Document 1), but if the permeability coefficient of the constructed low permeability soil varies, it will be 10 ^-7 m / s. There is a risk that a hydraulic conductivity of about s or less cannot be secured. If the constructed low-permeability soil cannot secure a hydraulic conductivity of 10 ^-7 m / s or less, high vacuum pressure (negative pressure of 60 to 65 kN / m ² or more in absolute value) can be applied to the clay layer. Therefore, a sufficient consolidation improvement effect cannot be obtained.

  In view of the above-mentioned problems of the prior art, the present invention provides a vacuum consolidation method capable of obtaining a sufficient consolidation improvement effect when performing a vacuum consolidation improvement on a ground having a mutual layer of sand and clay in the surface layer portion. And a vacuum compaction system.

In order to achieve the above object, the present inventors have obtained the following knowledge as a result of intensive studies. First, comparing the air permeability coefficient ka when air passes through the soil and the water permeability coefficient k when water passes, it is said that there is a relationship of the following equation (1) (non-patent document) 2).
ka = 70-80 × k (1)
According to the equation (1), it is understood that the water permeability coefficient k is considerably smaller than the air permeability coefficient ka, and the speed at which water penetrates the soil is much smaller than the speed at which air penetrates.

  Further, in the construction so far, the negative pressure is greatly reduced when air is sucked in the vacuum consolidation drain method, but when the water is sucked, there is almost no decrease in the negative pressure (see Non-Patent Document 3). The reason for this is that, as can be seen from equation (1), the amount of permeated water passing through the soil is much smaller than that of air, and air is inflatable, but water is incompressible. . Further, it is said that the airtight cap 12 of the plastic board drain material 11 must always be positioned below the water level as shown in FIG. 9 during the negative pressure action of the vacuum consolidation drain method (see Non-Patent Document 1). . This is to avoid the inflow of air into the drain material and to apply a high negative pressure to the clay layer.

  Therefore, a low permeable soil layer is formed on the surface layer of the ground, and water is supplied to the lower part of the low permeable soil layer so that the groundwater level of the ground is always above the airtight cap of the plastic board drain material. Thus, inflow of air into the drain material can be avoided, and thereby a high negative pressure can be applied to the clay layer to be improved.

  That is, the vacuum consolidation method for achieving the above object is a vacuum consolidation method for ground improvement for the ground where an alternating layer of sand and clay exists in the surface layer portion, and a drain material having an airtight cap at the upper end. From the ground surface to the ground, constructing a low permeable soil layer on the ground surface, water from the bottom of the low permeable soil layer into the ground, the groundwater level of the ground is more than the airtight cap Supply to be on top.

According to this vacuum consolidation method, a drain material is placed on the ground where the surface layer has a mutual layer of sand and clay to construct a low-permeability soil layer, and then water is supplied into the ground. Since the groundwater level is above the airtight cap of the drain material, it is possible to avoid the inflow of air into the drain material during vacuum consolidation. For this reason, when carrying out the vacuum consolidation improvement on the ground having a sand and clay mutual layer in the surface layer portion, a sufficient negative pressure can be applied to the ground, so that a sufficient consolidation improvement effect can be obtained. . Moreover, even when the low water permeability soil layer cannot secure the water permeability coefficient (about 10 ⁻⁷ m / s or less) necessary as an airtight seal layer, a sufficient consolidation improvement effect can be obtained.

  In the vacuum consolidation method, it is preferable to install a water supply unit on the ground surface before the construction of the low water permeable soil layer.

Further, the groundwater level is measured, and when it is determined that the groundwater level is lower than the depth position of the hermetic cap based on the measurement result of the groundwater level, the groundwater level is the hermetic cap. It is preferable to supply so that it may become above. Thereby, it can manage reliably so that a groundwater level may become above an airtight cap.

  Moreover, it is preferable to install a water-impervious wall in the ground so as to surround a range where a large number of drain materials are placed on the ground. As a result, the water supplied to the ground is blocked by the impermeable walls, so that it is possible to prevent the groundwater level of the ground to be improved from being lowered by spreading into the surrounding ground where the groundwater level is low. Can do.

  Moreover, it is preferable to apply the said vacuum consolidation method to the ground where at least a sand layer, a clay layer, and a sand layer exist in order from the ground surface as the mutual layer above the improvement target layer.

  A vacuum consolidation system for achieving the above object is a vacuum consolidation system for ground improvement for a ground having alternating layers of sand and clay on the surface layer and a low water permeable soil layer built on the ground surface. A plurality of drain materials placed on the improvement target layer in the ground, an airtight cap disposed at an upper end of each drain material, and one end connected to the upper end of each drain material together with the airtight cap. A drainage pipe, a vacuum device arranged on the other end side of the drainage pipe, and a water supply device arranged on the ground surface, and the ground from above the low water permeable soil layer by the water supply device. Supply water inside.

According to this vacuum consolidation system, after constructing a low-permeability soil layer on the ground with a sand and clay mutual layer in the surface layer and placing a drain material, water is supplied from the water supply device into the ground. Thus, since the groundwater level of the ground can be higher than the airtight cap of the drain material, it is possible to avoid the inflow of air into the drain material during vacuum consolidation. For this reason, when carrying out the vacuum consolidation improvement on the ground having a sand and clay mutual layer in the surface layer portion, a sufficient negative pressure can be applied to the ground, so that a sufficient consolidation improvement effect can be obtained. . Moreover, even when the low water permeability soil layer cannot secure the water permeability coefficient (about 10 ⁻⁷ m / s or less) necessary as an airtight seal layer, a sufficient consolidation improvement effect can be obtained.

  The vacuum consolidation system preferably further includes a groundwater level measuring device for measuring the groundwater level of the ground. Thereby, it can manage reliably so that a groundwater level may become above an airtight cap.

  In addition, by automatically controlling the water supply from the water supply device to the ground based on the measurement result of the groundwater level measurement device, the groundwater level is always above the predetermined value (above the airtight cap) Control and manage.

  According to the vacuum consolidation method and the vacuum consolidation system of the present invention, it is possible to obtain a sufficient consolidation improvement effect when the vacuum consolidation improvement is performed on the ground having a mutual layer of sand and clay in the surface layer portion.

It is a figure which shows roughly the vacuum consolidation system using the drain material with a cap which can perform the vacuum consolidation drain construction method by this embodiment. It is a principal part top view which shows roughly the vacuum consolidation system of FIG. It is a figure which shows roughly the water supply apparatus of FIG. 1, FIG. It is a principal part top view which shows roughly the injection hose of FIGS. 1-3. It is a figure which shows roughly the groundwater level measuring apparatus of the vacuum consolidation system of FIG. It is a block diagram for demonstrating the control system for keeping a groundwater level constant in the vacuum consolidation system of FIG. It is a flowchart for demonstrating each process S01-S10 of the ground improvement by the vacuum consolidation drain method of this embodiment. It is a graph which shows the particle size distribution by JIS A 1204 about the sandy soil which can be used in this embodiment, viscous soil, and these mixed soil. It is a figure which shows the conventional vacuum consolidation drain construction method roughly.

  Hereinafter, embodiments for carrying out the present invention will be described with reference to the drawings. FIG. 1 is a diagram schematically illustrating a vacuum consolidation system using a drained material with a cap capable of executing the vacuum consolidation drain method according to the present embodiment. FIG. 2 is a plan view of an essential part schematically showing the vacuum consolidation system of FIG. FIG. 3 is a diagram schematically showing the water supply device of FIGS. 1 and 2. FIG. 4 is a plan view of an essential part schematically showing the injection hose shown in FIGS. FIG. 5 is a diagram schematically showing a groundwater level measuring device of the vacuum consolidation system of FIG. FIG. 6 is a block diagram for explaining a control system for keeping the groundwater level constant in the vacuum consolidation system of FIG.

  As shown in FIG. 1 and FIG. 2, the vacuum consolidation system 10 is placed in a clay layer (improvement target layer) of the ground G from the ground surface S so as to suck pore water in the improvement target ground G. A plastic board drain material (PBD) 11, an airtight cap 12 connected to the upper end of each PBD 11, a drain hose 13 connected to each airtight cap 12, and drainage to depressurize the inside of each PBD 11 A vacuum pump P connected to the end portion side of the hose 13 via a water collecting pipe 14 and a header pipe 16, and a water supply device 20 for supplying water into the ground G in the direction a.

  Further, outside the large number of PBDs 11, a water-impervious wall 18 made of a steel sheet pile, a solidified continuous wall, or the like is placed on the ground G so as to reach the clay layer of the improvement target layer so as to surround the PBD 11 placement range. The Moreover, the low water-permeable earth layer 40 is built on the ground surface S after placement of PBD11. In addition, PBD11 and the airtight cap 12 can be set as the structure disclosed by patent document 1, for example.

  As shown in FIGS. 1 to 4, the water supply device 20 quantifies water to the water supply hose 21, which is a water supply unit disposed on the ground surface S before the construction of the low water permeability soil layer 40, and the water supply hose 21. A constant volume pump 22 to be supplied, an on-off valve 23, and a water tank 24 are provided. As shown in FIG. 4, the water injection hose 21 has a plurality of holes 21a for water injection and a filter 21b made of paper, nonwoven fabric, or the like joined so as to cover the holes 21a.

  Further, as shown in FIG. 5, the vacuum compaction system 10 includes a groundwater level measuring device 30, and the groundwater level measuring device 30 includes an upper pore water pressure gauge 31 installed at a relatively shallow position in the ground G, and a ground G And a lower pore water pressure gauge 32 installed at a relatively deep position.

The groundwater level H1 (distance from the ground surface S on the ground to be improved to the water surface WL1 in FIG. 5) can be measured by the groundwater level measuring device 30 in FIG. 5 as follows.
Hw = (u2-u1) / γw (1)
H1 = H0-Hw (2)
However,
u1: Measured value of upper pore hydrometer 31 u2: Measured value of lower pore hydrometer 32 γw: Unit volume weight of water Hw: Distance from lower pore hydrometer 32 to WL1 H0: Lower pore hydrometer 32 from ground surface S Distance to

  From the water supply device 20 in the direction a, the groundwater level H1 in the ground to be improved measured by the groundwater level measuring device 30 as described above is higher (higher) than the depth position of the airtight cap 12. Supply water to the ground. For example, as shown in FIG. 6, the measurement data of the groundwater level measurement device 30 is input to a personal computer (personal computer) PC, and the personal computer PC has the groundwater level H1 calculated from the measurement data by the formulas (1) and (2) as a predetermined value. 3 is operated (close to the depth position of the airtight cap 12), the constant volume pump 22 of the water supply device 20 of FIG. 3 is operated. With such a control system of FIG. 6, water is automatically supplied from the water injection hose 21 into the ground, and automatic control is performed so that the groundwater level H1 is always above the depth position of the airtight cap 12. it can.

  As shown in FIG. 1, the ground G to be improved in the present embodiment includes a sand layer, a clay layer, a sand layer, a clay layer, a sand layer, a thick clay layer to be consolidated, and a sand layer in order from the ground surface S. Thus, the ground G has an alternating layer in which sand layers, clay layers, sand layers, clay layers, and sand layers are alternately formed near the ground surface S, and the layers constituting the alternating layers are often relatively thin.

  Next, the ground improvement process by the vacuum consolidation drain method of this embodiment with respect to this ground is demonstrated with reference to FIGS. FIG. 7 is a flowchart for explaining the steps S01 to S10 for ground improvement by the vacuum consolidation drain method according to the present embodiment.

  First, the PBD 11 is placed on the clay layer to be consolidated in the ground as shown in FIG. 1 from the ground surface S by a PBD placing machine (S01). At this time, the PBD 11 is driven so that the airtight cap 12 at the upper end is located at a predetermined depth from the ground surface S. In this way, a large number of PBDs 11 are placed so that the minimum planar arrangement is, for example, a square of 1.0 m.

  Further, a water shielding wall 18 made of a steel sheet pile, a solidified continuous wall or the like is placed so as to reach the clay layer to be consolidated so as to surround a large number of PBD 11 placement ranges (S02).

  Next, the drain hose 13 is connected to the vacuum pump P through the water collection pipe 14 and the header pipe 15 as shown in FIG. 2 (S03).

  Moreover, the water injection hose 21 is arrange | positioned to the ground surface S of ground improvement object like FIGS. 1-4, and it connects to the constant volume pump 22 (S04). The vacuum consolidation system 10 of FIG. 1 is constructed as described above.

Next, sandy soil and cohesive soil are procured at the ground improvement target site, and a mixed soil is prepared by mixing these sandy soil and cohesive soil at a mass ratio of, for example, 2: 1. The particle size distribution according to 1204 is shown in FIG. As shown in FIG. 8, the content of fine particles in the mixed soil (mass ratio of 75 μm or less) was about 28%. When the water permeability test was performed on this mixed soil based on the “Soil Permeability Test Method” (JIS A 1218), the hydraulic conductivity k was 1 × 10 ⁻⁷ m / s.

  The above-mentioned sandy soil and viscous soil are stacked in two layers at a mass ratio of 2: 1. The stacked sandy soil and viscous soil are mixed with a backhoe or the like, and this mixed soil is mixed on the ground surface S in FIG. Then, a low-permeable soil layer 40 is constructed on the ground surface S by rolling out with a wetland bulldozer at a thickness of 1 m (S05).

  Next, the groundwater level H1 in the ground to be improved is measured by the groundwater level measuring device 30 in FIG. 5 (S06). When the groundwater level H1 is lower than a predetermined value (S07), water is supplied into the ground by the water supply device 20 of FIGS. 1 to 4 (S08).

  If it is confirmed that the groundwater level H1 in the ground to be improved is higher than the depth position of the airtight cap 12 in the above-described groundwater level measurement step S06, then the vacuum pump P is operated and the ground G is passed through the PBD11. A negative pressure is applied (S09). In this way, the pressure loading due to the negative pressure is performed on the ground G for a predetermined period while draining the pore water in the ground to the outside (S10). During this time, the measurement of the groundwater level H1 (S06) is continued, and it is always confirmed that the groundwater level H1 is above the depth position of the airtight cap 12.

  As described above, according to the vacuum consolidation drain method of the present embodiment, the PBD 11 is placed on the ground G having a mutual layer of sand and clay on the surface layer portion to construct the low-permeable soil layer 40, and then the water supply By supplying water into the ground G by the device 20, the groundwater level H1 of the ground G becomes higher than the airtight cap 12 of the PBD 11, so that inflow of air into the PBD 11 can be avoided during vacuum consolidation. For this reason, since a sufficient negative pressure can be applied to the ground G where the surface layer portion has a mutual layer of sand and clay, a sufficient consolidation improvement effect can be obtained when the vacuum consolidation improvement is performed on the ground. it can.

In addition, even if the low water permeability soil layer 40 cannot secure the water permeability coefficient (about 10 ^-7 m / s or less) required as an airtight seal layer, it is possible to avoid the inflow of air to the PBD 11, so that sufficient consolidation improvement effect can be achieved. Can be obtained.

  Further, the water supply from the water supply device 20 to the ground G is automatically controlled as shown in FIG. 6 based on the measurement result of the groundwater level H1 by the groundwater level measurement device 30 in FIG. Can be reliably managed so that is always above a predetermined value (above the hermetic cap).

  In addition, the impermeable wall 18 is placed and installed up to the clay layer to be consolidated on the ground G, and the impermeable wall 18 surrounds the numerous PBD 11 placement areas so that the water supplied into the ground G is impermeable. Since it is blocked by the wall 18, it can be prevented in advance that the groundwater level H1 of the ground to be improved is lowered due to diffusion into the surrounding ground having a low groundwater level (WL2) in FIGS. .

  As described above, the modes for carrying out the present invention have been described. However, the present invention is not limited to these, and various modifications can be made within the scope of the technical idea of the present invention. For example, in FIG. 6, the water supply from the water supply device 20 is automatically controlled based on the measurement result of the groundwater level H1 by the groundwater level measurement device 30, but the present invention is not limited to this, and the groundwater The water supply device 20 may be manually controlled based on the measurement result of the position H1, and this also ensures that the groundwater level H1 is always above a predetermined value (above the hermetic cap 12). Can be managed.

  In addition, in the ground G of FIG. 1, there are a sand layer, a clay layer, a sand layer, a clay layer, and a sand layer as mutual layers above the improvement target layer, but the ground to which the present invention is applicable is not limited to this, for example, It can be applied to the ground where at least a sand layer, a clay layer, and a sand layer exist in order from the ground surface as a mutual layer above the improvement target layer.

  According to the present invention, it is possible to obtain a sufficient consolidation improvement effect when performing a vacuum consolidation improvement on the ground having a mutual layer of sand and clay on the surface layer portion, so that the surface layer portion of the ground is composed of sand and clay. Since there are mutual layers, even if a sufficient negative pressure seal layer cannot be secured at the surface layer during vacuum consolidation, the ground improvement by vacuum consolidation can be carried out reliably.

DESCRIPTION OF SYMBOLS 10 Vacuum compaction system 11 Plastic board drain material, PBD, drain material 12 Airtight cap 13 Drainage hose 18 Impermeable wall 20 Water supply device 21 Water injection hose 30 Groundwater level measurement device 31 Upper pore water pressure gauge 32 Lower pore water pressure gauge 40 Low water permeability Soil layer G Ground H1 Groundwater level P Vacuum pump S Ground surface

Claims (6)

It is a vacuum consolidation method for ground improvement for ground where alternating layers of sand and clay exist in the surface layer part,
A drain material having an airtight cap at the upper end is driven from the ground surface to the ground,
Construct a low permeability soil layer on the ground surface,
A vacuum consolidation method for supplying water from the lower part of the low-permeability soil layer into the ground so that the groundwater level of the ground is above the hermetic cap.   The vacuum consolidation method according to claim 1, wherein a water supply unit is installed on the ground surface before the construction of the low water permeable soil layer. Measuring the groundwater level,
When it is determined that the groundwater level is lower than the depth position of the hermetic cap based on the measurement result of the groundwater level, the water is supplied so that the groundwater level is higher than the hermetic cap. The vacuum consolidation method according to claim 1 or 2.
  The vacuum consolidation method according to any one of claims 1 to 3, wherein a water-impervious wall is installed in the ground so as to surround a range where a large number of drain materials are placed on the ground. It is a vacuum consolidation system for ground improvement for the ground where alternating layers of sand and clay exist in the surface layer and a low permeability soil layer is built on the ground surface,
A number of drain materials placed on the improvement target layer in the ground;
An airtight cap disposed at the upper end of each drain material;
A drain pipe having one end connected to the upper end of each drain member together with the airtight cap;
A vacuum device disposed on the other end of the drain pipe;
A water supply device disposed on the ground surface,
A vacuum consolidation system for supplying water from the lower part of the low water permeability soil layer into the ground by the water supply device.   The vacuum consolidation system according to claim 5, further comprising a groundwater level measuring device for measuring the groundwater level of the ground.

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